The present invention relates to the field of electrolysis and specifically to controlling an electrolysis process to minimize power consumption without causing shorts through adjustment of anode/cathode spacing.
Many electrolysis processes are well-known and are employed, for example, in the production of sodium hydroxide, chlorine, potassium hydroxide, sodium sulfide and sodium hydrosulfite.
An electrolytic cell for the production of chlorine gas, for example, employs an anode immersed in a salt brine and positioned above a cathode formed by a thin layer of mercury on the cell bed. When current is passed through the cell, chlorine gas and sodium amalgam are produced, with hydrogen formation a competing reaction. The reaction for chlorine formation is given by the following expression: ##EQU1## For the usual range of temperatures, concentrations, and pressure, the reversible equilibrium potential (E.sub.r) for the reaction of Eq. (1) is -3.05 volts. Actual cell voltage drops are greater than E.sub.r because of cathode and anode polarization, and ohmic drop through the brine layer, and in electrical conductors and contacts.
The value of E.sub.r for the competing hydrogen reaction is lower than for the chlorine reaction, but high overvoltages kinetically limit hydrogen production rates during normal cell operation. Reduction of hydrogen overvoltage can result in a rapid increase in hydrogen concentrations in the chlorine gas and explosion hazards. Hydrogen overvoltage reduction will result from any of a number of causes, such as impurities in the brine or mercury, locally high current densities caused by poor current distribution, or by short circuiting between the cell anode and mercury cathode, across the brine gap (a "cell short," or "shorting").
With the electrolysis reaction proceeding in accordance with Eq. (1) the sodium-mercury amalgam formed is caused to flow to a decomposer where it is combined with water to produce sodium hydroxide, NaOH, as indicated by the following expression: EQU Na(Hg) + H.sub.2 O .fwdarw. NaOH + 1/2 H.sub.2 + (Hg) Eq. (2)
The power consumed in carrying out the electrolysis reaction of Eq. (1) is a product of the cell potential, Ec, which is the potential drop across the cell and the cell current, Ic, which is the current through the cell. In order to employ the minimum power, the cell potential, Ec, is reduced to the smallest value which will maintain the electrolysis reaction of Eq. (1) without shorting or permitting the competing hydrogen reaction to predominate.
The component potentials which are summed to form the cell potential, Ec, are: the reversible potential, the ohmic potential due to the potential drop across the electrolyte, contact potential due to the potential drop at the bus joints, the bus potential due to the potential drop along the cell conductors, and the polarization voltages due to the polarization at each of the two electrodes. Over the normal range of cell operation conditions, polarization and ohmic voltage drops can be represented as the sum of constant terms plus terms dependent on temperature (.tau.) and cell current (Ic). The reversible potential, plus the constant portions of the polarization and ohmic potentials can be represented by the constant Eo.
For convenience, the current-dependent terms of the contact and bus potential drop can be combined as a temperature-varying resistance (R.tau.) multiplied by the cell current (Ic), to yield (R.tau.) (Ic).
The current-dependent terms of the brine potential drop can be expressed as the product of a temperature-dependent brine resistance term (.rho..tau.) and the ratio of the brine gap (S) to the anode area (A) to yield (Ic) (.rho..tau.) (S/A).
With these definitions, the cell potential is given by the following expression: EQU Ec = Eo + [Ic][R.tau.] + [IC][.rho..tau.][(S/A)] Eq. (3)
In order to minimize the cell potential, Ec, the potential drop across the electrolyte, [Ic][(.rho..tau.) (S/A)] is reduced by reducing the anode/cathode spacing, S.
Although it has long been known to be desirable to reduce the anode/cathode spacing to reduce power consumption, such a reduction can increase the hazards of cell shorts. These are undesirable in that they often require stopping and restarting of the electrolysis reaction, in that they cause damage to the anodes, and in that they are accompanied by undesirable alternative and dangerous hydrogen generation which reduces current efficiency and may result in explosions and cell damage.
In order to regulate an electrolysis reaction by adjusting anode/cathode spacing, others have previously suggested moving the anode momentarily into contact with the cathode. Such anode/cathode contact, besides resulting in a short circuit, theoretically is supposed to identify a reference position where a short will occur. After the harmful short is detected, the anode is retracted a predetermined amount from that reference position. In actual practice, of course, the objective is to avoid shorts and therefore "reference positions" are established by operators who known, through experience, the location of the reference positions which do not cause shorts. Notwithstanding suggestions in the prior art, it is not believed practical or desirable to actually induce short circuits for control purposes. This conclusion is believed true even though attempts have been made to detect shorts rapidly and, after detection, to retract the anode before substantial damage has occurred. Such attempts even when retraction is accomplished during early stages of an anode/cathode short, have not proved entirely satisfactory nor reliably stable.
Prior attempts at reducing the frequency of shorts have also monitored various electrical signals such as changes in anode/cathode voltage or current, to detect short-causing conditions. In one example, short-causing transients, like those output from a poorly regulated power supply, have been monitored. Such transients can be characterized as current "spikes" which are generally not oscillatory and which have a very high frequency spectrum when observed by Fourier analysis. While such monitors may be useful where poor power supply regulation exists, they have not heretofore been satisfactory for reducing shorts where good power supply regulation is present.
While other attempts at regulating anode/cathode spacing have also been tried, heretofore, no reliable or completely satisfactory indicators of shorts have been available in the absence of actually causing or detecting the actual undesirable shorts.
In accordance with the above background, it is the objective of the present invention to provide a method and apparatus, capable of use in production facilities, for controlling electrolysis reactions with reduced power consumption and with less damage from actual anode/cathode shorts.